Synthesizers (e.g., phase locked loops (PLLs), frequency/phase synthesizers, and delay locked loops (DLLs)) have been serving as key components in different systems, and they have evolved from simple feedback loops to sophisticated architectures (e.g., integer-N, fractional-N, and all-digital). A reference oscillator or clock is sometimes included in synthesizer (e.g., PLL) circuitry, although this is not strictly part of the loop itself even though a reference signal from the reference oscillator is specified for operation of the PLL.
Most frequency/phase synthesizers are based around a PLL. Synthesizers are widely used in all forms of radio communications equipment today. These synthesizers are found in a variety of devices from cellular phones to all forms of wireless products and domestic radios and televisions to professional radio frequency equipment such as signal generators and spectrum analyzers.
Synthesizers not only offer high levels of stability and accuracy (determined by the reference, which is normally a crystal oscillator), they are also easy to control from digital circuitry such as microprocessors. Some drawbacks of synthesizers include noise, such as phase and/or frequency noise. For example, the noise may include input noise from a reference clock and a phase and frequency detector, as well as noise from an oscillator such as a voltage controlled oscillator (VCO). Some techniques improve noise performance by properly selecting a loop bandwidth. This approach, however, suffers from an intrinsic limitation. As the VCO frequency increases, the noise begins to dominate and becomes more difficult to suppress. Accordingly, it is desirable to develop techniques to overcome these drawbacks.